Electrical distribution systems, for example for distributing an electrical power supply through a building or industrial facility, are often subjected to harmonic currents generated by non-linear loads such as electronic equipment (including computers, adjustable speed drives (ASD), uninterruptable power supplies (UPS), power rectifiers, etc.) and equipment that uses different kinds of arc processes (including arc discharge lighting systems). These harmonic-generating loads generate various levels of conventional harmonics (5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th etc.) and, for single phase line-to-neutral non-linear loads, also zero phase sequence or “triplen” harmonics (3rd, 9th etc.) in the power distribution system, the harmonic spectrum depending upon the nature of the harmonic-generating load.
These harmonic currents create many problems in the power distribution system, including increased voltage total harmonic distortion level, reduced electromagnetic compatibility of the loads, reduced reliability of the power distribution equipment, increased power losses, reduced power factor, and other problems which are well known to those skilled in the art.
Prior art systems for mitigating harmonic currents fall into six basic types:
1. Power factor corrected (PFC) power supplies: In these systems the rectified current is continually adjusted to smooth the current consumption waveform. PFC's are relatively expensive devices and their applications are limited. Also, PFC's cannot be retrofitted for use with existing power supplies, and are not practical for use with large ASD's.
2. Active filters: These devices inject into the conductors between the power distribution system and the load, harmonic currents having a polarity opposite to those generated by the load, thereby neutralizing harmonic currents flowing into the power distribution system. Active filters have many disadvantages, including high cost, poor reliability. Active filters also are not practical for use with large ASD's.
3. Resonant L-C filters: L-C filters are commonly used in power systems, tuned to different harmonic frequencies to mitigate specific harmonic currents. These devices present many problems which are well known to those skilled in the art, including high cost and the tendency to cause the system to operate with a leading power factor. Further, because L-C filters are non-directional they are easily overloaded by untreated harmonic currents generated by other harmonic sources connected to the power distribution system (for example in a neighboring facility), resulting in overloading and frequent failures of the filter's capacitor bank.
4. AC chokes: In this harmonic mitigating technique reactors are connected in series between the line and the load. This technique is simple, reliable and relatively low cost, however it results in a high voltage drop across the reactors. To reduce the voltage drop one must reduce the choke reactance level, which commensurately reduces the effectiveness of the choke and substantially limits harmonic current mitigation.
The voltage can be boosted by connecting a capacitor bank between the load and the choke, but this frequently causes the system to operate with a leading power factor (especially in the case of light loading). In this case, since the reactance of the reactor at harmonic frequencies is much higher than the reactance of the reactor at the fundamental frequency, a large part of the harmonic currents drain through the capacitor. The capacitor has a high reactance at the fundamental frequency. However, the voltage drop across the choke remains very high. Thus, large compensating capacitors must be connected between the load and the choke to boost the voltage, which substantially increases the size and cost of the system and causes the system to operate at increased voltage levels during light loading conditions.
5. Phase shifting systems: Different kinds of phase shifters are available which allow the creation of quasi-multiphase systems, reducing certain harmonic levels. Harmonic currents of targeted orders are cancelled or substantially reduced depending upon the selected degree of the phase shift. However, such systems are typically limited in terms of the number of harmonic orders which can be mitigated, and the degree of harmonic mitigation depends upon the extent to which harmonics produced by the various harmonic sources are identical.
6. Passive wide-band filters, for example as described in U.S. Pat. No. 6,127,743 issued Oct. 3, 2000 to Levin et al., which is incorporated herein by reference and illustrated in FIG. 1 herein. This filter consists of a multiple winding reactor and a capacitor bank. This filter comprises a blocking coil (line winding 20 in FIG. 1) and filtering component comprising inductive and capacitive elements in a crosslink circuit. The filter described and illustrated in U.S. Pat. No. 6,127,743 is effective in eliminating a wide range of harmonics from the system.
However, since all inductive elements of the filter in U.S. Pat. No. 6,127,743 are wound on a common magnetic core, there is magnetic coupling between the various inductive elements of the filter. The harmonic currents flowing in the cross-link circuit generate a magneto motive force (MMF), and as a result of magnetic coupling between the blocking element of the filter (the line winding) and the cross-link circuit, the magneto-motive force (MMF) induces harmonic currents in the blocking element that flow directly into the power supply system, increasing the total harmonic current flowing into the power supply system and thereby reducing the effectiveness of harmonic mitigation by the filter.
Thus, in this solution, as more harmonic currents are diverted by the blocking element through the cross-link circuit, a greater MMF is generated and a higher level of harmonic current is thereby induced in the blocking element. With these additional harmonic currents, in order to improve filter performance to the required level the impedance level of the blocking element must be substantially increased. This causes a substantial voltage boost under no-load conditions, increased voltage drop at full load conditions, and sometimes drive stability problems especially when driving high-inertia loads. Also, different filter configurations have to be used for 1) rectifiers (drives) with no reactors; 2) rectifiers with DC line reactors; and 3) rectifiers with AC line reactors.
The sets of windings could alternatively be wound on separate cores, however there is a high cost to this both in materials used and in the space occupied by a multiple-core device (known as the ‘footprint’).